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A sound editor that I like is ChirpTone. For music I use BeepBox, so I recommend those two as they are pretty easy to use, especially because I don't have experience with audio composition.

Audacity is the go-to waveform editor for hobbyists if you already have samples and such.

Now it's implemented, if you still interested. 

For typical 8-bit sound effects, use FX Hammer. This tutorial will help you learn how to use it.

For more advanced sound effects, i.e. sample sounds, follow this tutorial. This is the sole reason I downloaded Audacity on my laptop.